A projection exposure apparatus (e.g., a stepper or the like) used in photolithography for manufacturing a semiconductor element, liquid crystal display element, or the likes transfers at a high precision a pattern formed on a master such as a reticle or photomask onto a substrate such as a wafer or glass plate coated with a photoresist via a projection optical unit. For this purpose, very high imaging characteristics are demanded for the projection optical unit, and a high measurement precision is demanded for, e.g., a laser interferometer for measuring the alignment of a stage which supports a substrate such as a master or wafer.
The imaging characteristics of the projection optical unit and the measurement precision of the laser interferometer are greatly influenced by changes in apparatus and ambient temperatures. The laser interferometer causes fluctuations of a laser beam upon a change in ambient temperature, degrading the measurement precision.
At the same time, a member holding a mirror as a measurement target of the laser interferometer deforms owing to the temperature change, the relative alignment of a substrate and the mirror serving as an alignment reference change, and the measurement precision decreases. Recently, demands have arisen for an alignment precision of a nanometer (nm) order. For example, even if a 100-mm thick low-temperature thermal expansion member (thermal expansion coefficient: 1×10−6) deforms by 100 nm upon a temperature change of 1° C., and the air temperature on the laser path of the laser interferometer changes by 1° C., the alignment measurement value may change by 100 nm depending on conditions. Hence, the temperatures of the building components of the projection exposure apparatus and its ambient temperature must be kept constant.
In a conventional projection exposure apparatus, a temperature rise of the apparatus by a heating member such as an exposure light source or a driving motor for driving a stage degrades the measurement precision of, e.g., the laser interferometer for measuring the stage alignment and the imaging characteristics of the projection optical unit.
In some cases, a temperature change of air changes the ambient temperature of the projection exposure apparatus, degrading the imaging characteristics of the projection optical unit. To prevent this, global air-conditioning is generally performed in which the projection exposure apparatus is stored in an environment control chamber, and temperature-controlled air is supplied into the chamber.
An exposure apparatus requiring precise temperature management undergoes temperature management by a combination of global air-conditioning and a method of directly supplying a temperature-controlled coolant such as air or water to a portion to be cooled. For example, to keep the measurement precision of the laser interferometer constant, air controlled to a predetermined temperature in a predetermined direction is supplied into a local space in the optical path of a laser beam between the laser interferometer and a mirror for reflecting a laser beam from the laser interferometer. To recover and remove heat generated by, e.g., a driving motor for driving a reticle stage or wafer stage, a cooling circulation pipe surrounds the driving motor, and a coolant such as water, air, or an inert liquid is circulated from an external temperature adjustment apparatus to the circulation pipe.
The temperature is controlled by setting a temperature sensor at or near a portion to be temperature-controlled, changing the flow rate or temperature of a coolant on the basis of an output from the temperature sensor, and adjusting the heat recovery amount (see Japanese Patent Laid-Open Nos. 7-302124 and 7-302747).
FIG. 14 is a view schematically showing an example of the driving device of an alignment stage in a conventional exposure apparatus. A wafer 501 is held by a top plate 503 of an alignment stage via a wafer chuck 502. A pattern formed on a master (not shown) such as a reticle is transferred onto the wafer 501 by irradiation light from an illumination optical unit (not shown) via a projection lens (not shown). The alignment stage aligns the wafer by relatively moving linear motors made up of a movable element 505 to which permanent magnets 506 are fixed and a stationary element 507 in which a plurality of coils 508 are buried, in accordance with driving signals from a controller 511 and driver 512. The movable element 505 is guided by hydrostatic bearings 524 and connected to linear motors 526 for vertical movement. The top plate 503 is set via the movable element 505 and linear motors 526. The stationary element 507 has a plurality of coils 508 and is constituted by a jacket structure so as to flow a coolant for recovering heat generated by the coils 508.
A mirror 504 is attached to the top plate 503, and the alignment of the top plate 503 is measured by an alignment measurement unit 516 such as a laser interferometer fixed to an alignment position where the unit 516 faces the mirror 504. A measurement value from the alignment measurement unit 516 is sent to the controller 511. The controller 511 controls the energization amount to the coil 508 of each linear motor via the driver 512 on the basis of the measurement value, drives and controls the linear motor, and drives and aligns the alignment stage at a high precision.
The stationary element 507 is connected to a coolant pipe 518 for circulating a coolant temperature-managed by a cooling unit 517, in order to prevent heat generated by each coil 508 upon driving the linear motor from conducting to air or a member and increasing the temperatures of the top plate 503 and wafer 501. The temperature-managed coolant drains heat generated by the coil 508 and is recovered by the cooling unit 517 outside the driving device. To compensate for the temperature, a temperature control unit 513 receives temperature data from a temperature measurement unit 515 for outputting temperature data measured by a temperature sensor 514 set on the movable element 505, and instructs the cooling unit 517 to control the temperature or flow rate of the coolant so as to minimize temperature changes of the movable element 505 and top plate 503. In addition, the temperature control unit 513 supplies to the linear motors via the coolant pipe 518 a coolant which is managed in temperature and adjusted in flow rate by the cooling unit 517. The temperature-managed coolant absorbs heat generated by the linear motor stationary element 507, and suppresses temperature changes of the movable element 505, top plate 503, and wafer 501.
In this prior art, to precisely manage the temperatures of a plurality of heating portions, {circle around (1)} a necessary amount of coolant temperature-controlled in accordance with the respective heating portions is supplied to the heating portions, or {circle around (2)} a coolant of the same temperature is supplied to all the heating portions after the flow rate is secured such that the coolant temperature after absorbing heat generated by all the heating portions is equal to or smaller than the allowable rise temperature of the apparatus. In {circle around (1)}, the pipe for supplying the coolant is complicated. Particularly to manage the temperature of the wafer stage or the like, problems such as a load resistance to driving due to the pipe rigidity and a location ensured to lay out the pipe must be solved. For example, to individually control the temperatures or flow rates of the respective linear motors when the linear motors have different driving patterns, the number of cooling units must be increased as the number of linear motors increases. Also, the number of coolant pipes extending from the cooling units to the alignment stage increases.
However, the number of pipes and their diameter are limited because a disturbance to alignment caused by the flexural rigidity or vibrations of the pipe must be suppressed. It is not, therefore, practical to arrange cooling units equal in number to the linear motors, individually lay out pipes from the respective cooling units to the respective linear motors, and control coolant amounts to the respective linear motors. For this reason, a given number of linear motors is set as one group, like {circle around (2)}, and controlled at the same coolant temperature or flow rate by using one cooling unit. It is difficult to execute precise temperature control for each linear motor. In this method, the cooling amount of the coolant is determined in correspondence with a portion having the largest heat generation amount, and an unwanted cooling amount (flow rate or temperature) of the coolant is inefficiently supplied to another heating portion having a small heat generation amount.
To rapidly cope with a change in the heat amount of a heating element such as a coil, there is proposed a method of predicting the heat amount of the heating element by a temperature control unit and controlling the heat recovery amount of a coolant. The coolant pipe extending from the cooling unit 517 to each linear motor is as long as 5 m or more. Thus, (1) even if the coolant temperature is controlled, a long time is taken to reflect the coolant temperature on each linear motor, and temperature control is delayed. (2) Even if the coolant temperature is controlled by the cooling unit 517 at a high precision, a high-precise temperature is not reflected when the coolant reaches the linear motor owing to movement of heat during a long pipe. (3) A large time lag occurs because the cooling unit 517 cannot change the coolant temperature as fast as an output from the linear motor. These problems make it difficult to perform high-precision temperature control for objects to be temperature-controlled such as a top plate and a substrate including a wafer to be aligned.
If the temperature is controlled based on an output from the temperature sensor, the output from the temperature sensor is changed after the temperature changes, so high-response temperature control cannot be achieved as a whole. Furthermore, attaching the temperature sensor increases cost and decreases reliability.
As an output from a recent exposure apparatus increases, the heat amount of each driving portion increases. It becomes difficult for the conventional method to ensure a coolant flow rate at which all generated heat is recovered and a temperature rise of a coolant is suppressed to be smaller than the allowable temperature difference of the apparatus. In other words, to ensure a high coolant flow rate, the pipe must be made thick under limitations on the pump ability or the like. Such a pipe is difficult to lay out. In addition, the thick pipe acts as a nonlinear driving load resistance with respect to an alignment driving portion and degrades the alignment precision.
Vibrations caused by the flow of a coolant along with an increase in coolant flow rate cannot be ignored and may adversely influence an alignment precision, which must be high. A coolant having a large heat capacity may be used to recover generated heat without excessively increasing the coolant flow rate. However, there is no coolant having a heat capacity with which heat generated by the driving unit of the exposure apparatus or the like can be recovered at a proper flow rate.
As described above, heat generated in the exposure apparatus has conventionally been recovered to suppress a temperature rise in order to suppress a temperature change in the apparatus. If heat generated in the entire exposure apparatus increases, the conventional method cannot completely recover the generated heat, and each portion of the apparatus inevitably changes in temperature. Even if generated heat can be completely recovered, the alignment precision degrades, which is in conflict with the purpose of increasing the alignment precision.